This invention relates generally to three-phase load centers and more particularly to the construction and placement of bus bars in three-phase load centers to save space.
There are numerous ways to construct load centers which are to be used for heavy industrial applications where the size of a load center is not a limiting factor. However, in light industrial and residential applications, there are limitations on the size of a load center. For example, in residential applications, it is desirable to place the load center in a wall cavity which typically is formed between wall studs which are spaced apart about 16" center to center. Likewise, in light industrial applications it is also desirable to place the load center in a wall cavity where the wall has a nominal depth of approximately 4 to 41/2" or so.
In the past, the bus bars in a load center and the circuit breaker plug-in units were connected to the bus bars by bolts or screws. This was feasible since the bus bars were constructed of heavy duty copper which has a square or rectangular cross-sectional configuration thick enough to accept bolts or screws. The bus bars were massive. The mass was required to minimize a load current density thereby maintaining a lower operating temperature of the bus bars. By making the bus bars long enough, the temperature stays low enough that the bus bars can be encapsulated in insulating material which reduces the space required between the bus bars and the metal enclosure of the load center. Also, where insulation is used on the bus bars, it is necessary to increase the mass of the bus bars to compensate for the poor heat transmission accompanied by insulation. Needless to say, the massive conductors required a great deal of conducting material such as copper which increases the expense of the load center. Also, circuit breaker plug-in units were attached by screws or bolts which is a labor intensive operation furthe increasing the cost of a load center. To minimize manufacturing costs, it is advantageous to use an insulative molded or extruded base pan upon which to mount the bus bars for the load center.
The modern molded or extruded base pan eliminates the need for many fabricated components and reduces assembly time. One such extruded base pan is disclosed in U.S. Pat. No. 4,449,296 which issued May 22, 1984 to Roger D. Luke and John M. Rhodes for "Method of Forming Electric Distribution Panel". The construction disclosed therein uses bus bars which are not square or rectangular in cross-sectional configuration but more closely resemble a flat piece of metal. The flat piece of metal is much easier to use in the manufacturing process than a rigid rectangular metal bar and reduces manufacturing costs. However, the structure illustrated in U.S. Pat. No. 4,449,296 is for a single phase load center; thus, it will be appreciated that it would be highly desirable to provide a three-phase load center which utilizes an extruded base pan and bus bars constructed of flat pieces of metal.
Naturally, as thin flat pieces of metal are incorporated into load centers for bus bars, the cross-sectional area of the bus bar decreases which increases the current density thereby generating more heat and becoming a limiting factor in the design of the load center. Thus, if flat bus bars are used, the heat must be eliminated in some manner. Typically, the heat is eliminated by increasing the spacing in the load center enclosure to foster the circulation of air for cooling. Naturally, this increases the depth of the load center so that it will not protrude beyond the wall cavity which is undesirable. While the insulative base pan does much to reduce the depth of the load center by eliminating the need for some spacers and insulators, an insulative pan is not enough. Accordingly, it will be appreciated that it will be highly desirable to provide a three-phase load center for residential and light industrial applications which has a shallow depth and is easy to manufacture and assemble.
Accordingly, it is an object of the present invention to provide a three-phase load center which has a shallow depth.
Another object of the invention is to provide a three-phase load center which is easy to manufacture and assemble.
Another object of the invention is to provide a load center which provides a means for cooling.